1. Field of the Invention
The present invention relates to an optical film, and more particularly, to an optical film suitable for use in a light-reflecting substrate having a thin metal film coated on a surface thereof and used for a reflection type liquid crystal display apparatus or a joint reflection/transmission type liquid crystal display apparatus.
2. Description of the Related Art
Hitherto, liquid crystal display apparatuses have been widely used, in particular, for displays of small-to medium-sized apparatuses, this being due to characteristic features such as it being possible to make the apparatus thinner and more compact than with a conventional CRT display, it being possible to operate the display at a low voltage, and it being possible to reduce electrical power consumption and thus save energy. Moreover, of liquid crystal display apparatuses, in particular reflection type liquid crystal display apparatuses, which use incident light from outside as a display light source, are constructed, for example, from a liquid crystal layer, and a light-reflecting substrate having an optical film coated with a thin film of a metal such as a silver alloy. A conventionally required backlight is not required, and hence such reflection type liquid crystal display apparatuses can be made light and thin, and are thus widely used as liquid crystal display apparatuses.
For such liquid crystal display apparatuses, in recent years, from the standpoint of further improving the visibility, there have been demands to reduce the optical absorptivity of the light-reflecting substrate and improve the durability under harsh environments such as an oxidizing atmosphere. Various research has been carried out into fulfilling these objectives.
For example, for an optical film constructed from three oxide thin film layers and two silver thin film layers are formed alternately on a major surface of a glass substrate, by making the two silver thin film layers have a stable crystal orientation, the stability of the silver thin films under high temperature and high humidity can be improved (see, for example, Japanese Laid-open Patent Publication (Kokai) No. 2000-056127).
Moreover, another optical film is constructed from a heat-reflecting metal film made of a silver-based metal formed on a transparent substrate, and a transparent nitride film made of a nitride of at least one metal selected from the group consisting of silicon, boron and aluminum formed on the heat-reflecting metal film, whereby oxidation of the heat-reflecting metal film is suppressed by the transparent nitride film (see, for example, Japanese Laid-open Patent Publication (Kokai) No. H06-345491).
Furthermore, another optical film is constructed from a transparent high refractive index layer made of indium tin oxide (hereinafter referred to as “ITO”) formed on a major surface of a transparent substrate, a thin metal film formed on the transparent high refractive index layer, and another transparent high refractive index layer made of ITO formed on the thin metal film, whereby the environmental durability of the thin metal film is increased by the adjacent transparent high refractive index layers made of ITO (see, for example, Japanese Laid-open Patent Publication (Kokai) No. H09-272172).
However, of these optical films, in particular for the optical film having thin metal films that are stabilized through their crystallinity, there is a problem that in an oxidizing atmosphere, the effect of the redox potential on material values is large, and hence the durability of the thin metal films drops. Furthermore, for the optical films having a transparent nitride film made of a metal nitride or transparent high refractive index layers made of ITO, it is difficult to obtain desired optical properties due to the overall thickness of the optical film becoming high, and moreover there is a problem that the thin metal film is exposed to the atmosphere due to minute pinholes present in the transparent nitride film or the transparent high refractive index layers made of ITO, and hence the durability of the thin metal film drops.